A3 kg toy car rests st the top of a 0.45 meter ramp. what is its gravitational potential energy?

gravitational potential energy : P
Gravity : g
Mass : m
height : h

P = mgh  = 3 x 9.8 x 0.45 = 13.23 Joule

Potential energy is work , from the known formula

W = Fd ( work = force x distance ) 

W = P ( in case of potential energy height change)

F is the force acting on the body in case of ideal ramp , the only force acting is the weight of the body

F = mg ( not just <m> as the force is mg (Newton) gravity effect)

d is the displacement in direction of force, as we have considered the force to be the weight not it's component in direction of the ramp , the change in displacement is the change in height so

d = h 

W = Fd = (F = mg) x (d = h) = mgh 

W = mgh = P